Phase-change memory (a.k.a., “PCM” or “PRAM”) is a type of nonvolatile memory that stores information using phase change materials. Phase change materials can change between a crystalline state and amorphous state. The crystalline state has a low electrical resistance in comparison to the amorphous state. To change from the crystalline state to the amorphous state, a current is passed through a phase change material to melt it via internal joule heating, and then a quench is performed. A change from the amorphous state to the crystalline state involves driving a phase change material to electrical breakdown, and annealing it using electrical current. Data can be stored and read from the phase change material based on its programmed electrical resistance.
By the above process, multiple bits of information may be stored in a single storage cell of a phase change material to provide a “multi-bit phase change memory” (i.e., “multi-bit PCM”). In a multi-bit PCM, each storage cell can be set with a resistance state selected from a range corresponding to various intermediate states between the crystalline state and the amorphous state. For example, different resistance states (e.g., R1, R2, R3, and R4) can represent to different binary values (e.g., 00, 01, 10, and 11).
Current implementations of multi-bit PCMB use a combination of analog-to-digital converters (ADCs) and digital to analog converters (DACs). However, these implementations are very slow for reading and even slower for writing. In addition, the ADCs and DACs require a large amount of chip area.